This invention relates to non-crosslinked, saturated ethylene polymers having maleic anhydride grafted thereto and a process for preparing such polymers.
The reaction of maleic anhydride with polymers is well known in the art. The thermal "ene" reaction between an unsaturated polymer and maleic anhydride has been widely used. U.S. Pat. No. 2,973,344, issued Feb. 28, 1961 to Fasce, discloses the preparation of modified polyolefins by heating low pressure polyalkene polymers which possess non-terminal unsaturation with maleic anhydride, in the presence or absence of either an inert hydrocarbon solvent and/or a peroxide catalyst. U.S. Pat. No. 3,884,882, issued May 20, 1975 to Caywood, Jr., discloses the preparation of adducts of maleic anhydride and unsaturated elastomeric ethylene-propylene-unconjugated diene terpolymer by reaction at elevated temperatures in the absence of a radical catalyst, e.g., an organic peroxide. The presence of a radical catalyst results in the crosslinking of the elastomer.
The grafting of maleic anhydride onto saturated polymers in the presence of free radicals, either generated by shearing or by heating free radical precursors such as organic peroxides, is also well known in the art. However, reaction in the absence of a solvent results in crosslinking in, e.g., polyethylene or ethylene-propylene copolymers. U.S. Pat. No. 3,236,917, issued Feb. 22, 1966 to Natta et al., discloses that the grafting of maleic anhydride onto saturated copolymers of ethylene and an .alpha.-olefin, in the presence of a radical initiator at elevated temperatures and in the absence of a solvent, results in the grafting of maleic anhydride onto two chains, thereby crosslinking the copolymers. U.S. Pat. No. 3,862,265, issued Jan. 21, 1975 to Steinkamp et al., discloses the controlled degradation of polyolefins in an extruder in the presence of a radical catalyst, and the concurrent grafting of maleic anhydride onto the base polymer.
Gabara and Parejko (Journal of Polymer Science, A-1, 5, 1547 (1967)) reported that when maleic anhydride is grafted onto low density polyethylene film suspended in an acetic anhydride solution containing a free radical catalyst, the film becomes crosslinked. However, reaction in a solvent such as xylene results in appendage of maleic anhydride without crosslinking the polyethylene (S. Parejko, W. Gabara and J. Kulesza, Journal of Polymer Science, A-1, 5, 1563 (1967)), U.S. Pat. No. 3,873,643, issued Mar. 25, 1975 to Wu et al., discloses that after mixing high density polyethylene with maleic anhydride in the presence of a radical catalyst in a Brabender Plasticorder at elevated temperatures, the melt index of the polymer is significantly reduced.
Attempts have been made to prevent or avoid crosslinking during the reaction of maleic anhydride with polymers. For example, solvents have been used to prevent crosslinking of the polymer during this reaction, but the use of solvents requires the use of equipment for solution reactions, and the separation and recovery of the solvent as well as the polymer.
When the reaction is conducted in the absence of solvent, the reaction results in very high levels of crosslinking. Such crosslinked products are particularly troublesome because these products do not have good flow properties and are, therefore, not useful in extrusion applications. On the other hand, grafted products are preferably prepared in an extruder in order to achieve higher product quality as well as to minimize costs. However, extruder reactions are by necessity done in the absence of large amounts of solvent. (A small amount of solvent may be used to introduce, e.g., a catalyst into the extruder, though.) Thus lies the heart of the problem. In order to obtain the benefits of the products prepared in an extruder, it is necessary to avoid the use of solvents; on the other hand, the absence of solvents results in levels of crosslinking which render the product unacceptable for extruder applications.
U.S. Pat. No. 4,506,056, issued Mar. 19, 1985 to Gaylord, discloses a process for preparing carboxyl-containing polymers which involves mixing together maleic anhydride, a free radical initiator, an additive which inhibits the homopolymerization of maleic anhydride but not that of methyl methacrylate below about 100.degree. C., and a polymer, above its melting point, in the absence of solvent. Examples of the additive are various nitrogen-, phosphorous- and sulfur-containing compounds. While the use of such additives results in products with little or no crosslinking, the presence of these additives in the resulting grafted polymers can cause odor problems, lead to discoloration of the polymer, provide undesirable polymer degradation mechanisms, and can result in toxicity problems, especially where the product polymer is used in food packaging.
A particularly useful copolymer that finds application in foodwraps, etc. is a copolymer of ethylene and methyl acrylate. When grafted with a grafting agent such as maleic anhydride, the resulting product has enhanced adhesion properties which not only allows it to be bound to aluminum, but additionally allows for printing to be done on its surface.
In view of the above, it is apparent that there is a need to develop a process which can prepare substantially non-crosslinked grafted ethylene polymers, such as ethylene-methyl acrylate copolymers, in the absence of solvent and without the additives used in the prior art.
It has now been discovered that crosslinking can be decreased during the free radical initiated reaction, in the absence of solvent, of saturated ethylene polymers and maleic anhydride if the reaction is conducted in the presence of a polyamide.